Not sure why you were downvoted. But in response to your query, there is always a risk of someone obtaining more than 50% of the netwrok, however with that come

Not sure why you were downvoted. But in response to your query, there is always a risk of someone obtaining more than 50% of the netwrok, however with that comes cost, to attack the blockchain by misusing your 50% of the network causes as much damage to yourself as it does to anyone else. Right now if you contolled 50%+ you could double spend your coins, or perhaps you could swap a load of shares into your addresses if NASDAQ were using it to track share ownership, however if you did how long do you think it would take for them to shut down using the blockchain and use e.g. a centralised vrsion they run themselves, you then lose a lot of value in bitcoin, of which you ar emining over 50% of the daily allowance. Yes it would be possible but it would be expensive and a very short lived way of making money. of course a nation state that wanted to get rid fo BTC would like this but for private individuals or firms there would be no incentive to under take an attack like this.